1. Field of the Invention
The invention relates to the field of optical wavelength selection to enable optical energy to be processed selectively as a function of wavelength. In particular, the invention provides a tunable narrow band polarization encoder.
According to an inventive aspect, a Fabry-Perot element (an xe2x80x9cetalonxe2x80x9d comprising two reflectors spaced along a beam path) is used in a reflective mode, with the cavity between the reflectors containing a preferably-controllable birefringent material. The second reflector can be highly reflective, such that all or a high proportion of the incident light energy is reflected back along the path of the incident beam. The birefringent material imparts a polarization transformation exclusively to the resonant wavelength component (or components) of the input beam.
The resonant components have an effective optical path between the first and second reflectors of the Fabry-Perot element or etalon that is equal to, or has an integer multiple that is equal to, half the resonant wavelength. The entire input beam could be reflected by a reflective Fabry-Perot, but the polarization transformation is applied to the wavelength(s) that resonate in the cavity. The situation is such that most or all of the light is reflected, but the nonresonant component is reflected at the first reflector and the resonant component is affected by the cavity.
A birefringent material can be disposed in the cavity. A birefringent material has two refractive indices, ne and no. An incident beam that is aligned on an angle xcex8 can be divided into orthogonal polarization components parallel to the ne and no birefringent axes. The polarization aspect adds another factor because one of the different polarization components may be resonant while the other is not.
In any event, a polarization transformation can be arranged according to the invention to produce a polarization condition that thereafter can be used as a criterion to discriminate or separate the wavelength from the remainder of the beam, for example using a polarization filter or beam splitter. This has some very useful applications.
According to a further inventive aspect, control of a birefringent element in the cavity, such as applying a control voltage to a liquid crystal birefringent element, adjusts the effective optical length of the path between the reflectors, at least for one polarization axis aligned to ne or no of the incident beam, permitting tuning for selection of the resonant wavelength that undergoes polarization transformation.
The first reflector is preferably highly reflective and the second reflector can be substantially completely reflective (e.g., over 99%), or perhaps allowing a small amount of light to pass through to a monitoring sensor. The reflectors turn a high proportion of the light energy backwards along the path of the incident beam. However the resonant wavelength of the Fabry-Perot etalon has an altered polarization orientation (rotated 90 degrees in an exemplary embodiment described below). This distinct polarization attribute marks the resonant wavelength and provides an aspect whereby it is possible further along the reflected beam path to discriminate for that wavelength.
A number of specific applications of such a reflective Fabry-Perot element are disclosed for distinct polarization marking or similarly facilitating selective processing of particular wavelengths. The elements can be employed with polarization diversity arrangements such as crystals, polarization beam splitters, half wave plates and the like, to achieve given conditions such as a predetermined reference polarization orientation in the incident beam, whereby the polarization transformation produces a detectable difference in the affected component, namely the resonant wavelength. The device can be pixilated or otherwise laterally subdivided for parallel processing of plural beams such as beams carried by separate optical fibers. The devices can be cascaded such that plural wavelengths are selected in turn, and each is distinctly polarized. These wavelengths can then be discriminated, separated or otherwise handled independently due to their distinct polarization.
An inventive Fabry-Perot element is provided with birefringent material in a cavity between at least two surfaces that are each at least partly reflective. The second reflector (or the last of a cascaded series) is preferably substantially fully reflective. The reflecting surfaces are spaced by a distance equal to one or more integral half wavelengths of a particular resonant wavelength. The action of a Fabry-Perot etalon usually is to pass light at the resonant wavelength through two spaced reflectors and to reflect other wavelengths, i.e., as a narrow bandpass filter. In the reflective mode according to the invention, the two reflective surfaces introduce a xcfx80 phase shift between two reflected beams, namely one reflected from the front reflective surface and the other from the cavity. The cavity between the reflectors contains the birefringent material.
The Fabry-Perot element can be tunable. Controllable liquid crystals preferably are used in the cavity for at least part of the birefringent material. Control of such crystal by application of a control voltage alters the effective birefringence, and is used according to the invention to adjust the effective optical distance between the reflectors as applied to one of two mutually perpendicular axes of the incident beam.
In an exemplary arrangement, the input beam can be plane polarized and oriented at 45 degrees to the fast axis of the birefringent material. The selected wavelength, namely the resonant wavelength of the beam incident on the Fabry-Perot element, undergoes a polarization transformation that the remainder of the beam does not undergo. In the example, the selected wavelength undergoes a 90 degree polarization rotation for linearly polarized light at 45 degrees with respect to the fast axis of the birefringent material in the cavity. In this example, the orientation of the plane polarized input beam is such that part of the incident light energy is affected by the polarization transformation, and the adjustable birefringence of the birefringent material permits tuning for selection of the resonant wavelength. Other arrangements and orientations are possible that allow the controllable birefringence to operate on a selected wavelength of the input beam, provided that the necessary conditions are present (e.g., the resonant wavelength must be present in the input and appropriately aligned, etc.). For example, the tuning arrangement can be coupled to other tuning arrangements, such as piezoelectric tuning controls for adjusting spacing, electroclinic crystal controls for adjusting the orientation of the birefringence, etc.
A number of additional aspects will become apparent in the detailed description and the examples of specific optical processing steps and elements that appear in the detailed disclosure below.
2. Prior Art
It is sometimes advantageous to separate light at one or more wavelengths from light at other wavelengths as the plural wavelengths propagate together as a group. For this purpose, a tuned or tunable wavelength-separating device is needed. Without limitation, such tuned or tunable wavelength selective devices might be helpful, for example, to enable comparative measurement of energy at a range of wavelengths (i.e., spectral content), to filter light used for illumination or reflected from a target, for modulating or demodulating specific wavelengths in wavelength modulation or in wavelength division multiplexed optical communications, and other applications. In connection with optical fibers, a primary application of such devices is in connection with applying information signals to light and extracting the information signals thereafter (modulating and demodulating). Such mechanisms require some sort of technique or mechanism by which certain wavelengths or bands are treated differently than others.
Optical wavelength division multiplexing (WDM) can provide substantial bandwidth over a signal path, for example carrying light on an optical fiber with information bearing signals transmitted simultaneously at several different wavelengths over the same optical fiber. Wavelength selective elements are used or are advantageous for purposes such as selecting a wavelength band to be modulated (by amplitude or wavelength), from a broad band or other multiple wavelength signal, or selecting a band to be demodulated, or actually modulating and/or demodulating at the selected wavelength or within a bandwidth around a center wavelength.
There are a number of potential methods for separating light at multiple wavelengths into discrete beams of the constituent wavelengths. Wavelength-dependent separating devices can range, for example, from simple refractive prisms to complex spectrometers with diffraction gratings. Such devices can cause a beam to diverge as constituent wavelengths directed at different angles, whereby the constituent wavelengths can be discriminated, directed or sensed. Other devices also are known, such as waveguide demultiplexers and multiplexers and acousto-optic devices.
Fabry-Perot devices are a known form of transmissive filter that passes only a specific wavelength determined by the resonant distance between two reflectors, and blocks other wavelengths, typically reflecting the blocked wavelengths back in the opposite direction from incident beam. Between the two spaced reflectors of a Fabry-Perot device, the light energy is reflected back and forth between the reflectors. The light energy interferes and at specific wavelengths produces a standing wave. Where the space between reflectors is equal to the length of a standing wave (precisely one half wavelength, or precisely an integral number of half wavelengths), incident light directed normal to spaced parallel reflector planes can pass through the two spaced reflectors. All other wavelengths are effectively blocked.
Therefore, a parallel spaced reflector pair or xe2x80x9cFabry-Perotxe2x80x9d element or etalon may be useful as a filter to pass only a predetermined set of wavelengths. The Fabry-Perot structure, and in particular the reflector spacing, may be chosen to support resonance at plural wavelengths that are present, or the structure can be chosen to eliminate higher order modes. In a real world device, a Fabry-Perot passes a limited passband. The bandwidth is determined by dimensional variations of the cavity between the reflectors and by reflectivity. The narrowness or broadness of the passband is associated with a characterizing factor known as xe2x80x9cfinesse.xe2x80x9d
A Fabry-Perot element could consist of a static pair of reflectors rigidly spaced by an air gap, thus passing a fixed wavelength (or assuming integer multiples, a fixed set of wavelengths). A Fabry-Perot may be arranged to filter for a selected tunable wavelength by providing a means to change the effective optical cavity length between the reflectors in one way or another. The cavity length might be changed by moving either or both of the parallel reflectors toward or away from one another. Alternatively, the physical spacing between the reflectors could be unvarying, but the index of the material in the cavity might be changeable. For example, it is known to use controllable liquid crystals in a cavity of a light transmissive Fabry-Perot, and to select the wavelength that is transmitted by changing the index of the liquid crystal in the cavity.
Known Fabry-Perot devices necessarily are transmissive rather than reflective. There is no reason to select a particular wavelength if that selected wavelength is immediately recombined with the wavelengths that were not selected. Such recombination occurs inherently if the Fabry-Perot is reflective rather than transmissive. The point of Fabry-Perot devices is to separate a selected bandpass wavelength from other wavelengths and to allow only the selected wavelength to pass. The passed wavelength(s) is (are) directed along an optical path that is different from the paths of other wavelengths that are not selected. If one considers a reflective device of this type, the effect would be to direct the selected wavelength along the same optical path as the non-selected remaining wavelengths, thus making no change in polarization properties. Any benefit associated with selecting a particular wavelength would be lost when the selected and unselected portions were recombined.
A Fabry-Perot element is resonant at a particular wavelength because its reflectors are spaced to correspond to that wavelength (actually one half-wavelength) and a standing wave is produced between the reflectors. Wavelengths other than the resonant wavelength are reflected but the resonant wavelength is passed or transmitted through the element or etalon. The element is a narrow passband filter that passes only its resonant wavelength. An example of a Fabry-Perot etalon having this narrow bandpass characteristic is disclosed, for example, in U.S. Pat. No. 5,321,539xe2x80x94Hirabayashi et al. That patent also discloses using a liquid crystal material in the cavity between the Fabry-Perot etalon reflectors. A liquid crystal material can be oriented along a so-called buffing or rubbing axis and controlled anisotropically, by application of an electric field. The liquid crystal defines a different optical path length along mutually perpendicular axes for mutually perpendicular polarization components of an incident beam. In this manner, polarized light aligned at the required orientation, or at least having a vector component at the required orientation, can be caused to traverse an optical path having a length that can be modified by an electrical signal. With reference to the Fabry-Perot etalon, the resonant wavelength is changeable along that path and thus the resonant wavelength can be electrically selected. The device can be tuned.
This aspect of a Fabry-Perot element is useful in a bandpass application to pass only the selectively tuned resonant wavelength desired. Whether or not tuned, for the reasons discussed above, there is no apparent use for such a characteristic in a reflective device rather than a transmissive one. Whether or not tuned, the selected resonant wavelength would be recombined with the non-selected wavelengths in a reflective device. Whether tuned or not, any benefit associated with the selection or discrimination is lost because the wavelengths are recombined.
Various devices have been produced to take advantage of the selective nature of a Fabry-Perot element in a forward or transmissive direction. Examples include, for example, U.S. Pat. Nos. 5,068,749, 5,111,321 and 5,150,236, all to Patel; and 5,452,127xe2x80x94Wagner, the disclosures of which are hereby incorporated.
It is an aspect of the present invention that one or more wavelengths of a beam are marked selectively. Although the marked and unmarked wavelengths may be recombined, the marking is useful as a characteristic to select for the presence of that wavelength or to divert that wavelength, using a beam splitter, prism, grating or the like.
Examples of efforts in the areas of combining and separating wavelengths include U.S. Pat. Nos. 6,154,591xe2x80x94Kershaw, 6,208,444xe2x80x94Strong et al. and 6,222,958xe2x80x94Paiam. In another example, in U.S. Pat. No. 6,125,220xe2x80x94Copner et al., transmitted and reflected beams exiting a partly reflective etalon are processed and recombined.
What is missing in the prior art is a simple and effective way to exploit the resonant cavity aspects of a Fabry-Perot or another similar resonant optical cavity device (such as a ring resonator) in a wholly reflective way. The prior art lacks an effective way to apply an incident beam to an optical element that marks one or more preferably-tunably-selected resonant wavelengths in a way that permits those specific wavelengths to be discriminated, and returns the whole incident beam, including those wavelengths (now distinctively marked) and the other wavelengths as well.
It is an object of the invention to provide a polarization-based method and apparatus wherein an optical interferometer contains a birefringent material for producing a wavelength-dependent polarization transformation.
It is an object of the present invention to provide a polarization-based mechanism in association with a Fabry-Perot or similar resonant optical structure employed in a reflective mode, with a polarization transformation provides a distinctive marking aspect whereby the resonant wavelength can be separated from the remainder of the reflected light, to provide useful devices. This mechanism, preferably comprising a Fabry-Perot structure containing controllable birefringent material for tuning, produces selective narrow band polarization that is useful as an encoding device and can be used as a tunable wavelength selective device and in other ways that are discussed in this disclosure.
The invention is a tunable narrow band polarization encoder that introduces a xcfx80 phase shift between two orthogonal states of polarization of a selected narrow band in an input beam. Effectively the invention behaves like a narrow band tunable half wave plate.
In some more particular arrangements, the invention is selective by virtue of the input beam orientation. In some of the disclosed examples, the input beam is plane polarized and a wavelength selective arrangement alters the differential phase between the components corresponding to the two orthogonal birefringence axes, which results in a polarization transformation of that wavelength.
The invention is applicable to a general case in which a single wavelength of a plane polarized input beam is marked by polarization transformation and diverted by a polarization sensitive beam splitter. A number of other arrangements are also disclosed, including for example arrangements to ensure a particular input beam polarization orientation or to process the polarization transformed output. In one example the Fabry-Perots or optical resonance elements are cascaded whereby plural selected and preferably-tuned wavelengths are marked with polarization transformations permitting the marked components to be discriminated.
These and other objects are met according to the invention in a Fabry-Perot optical resonance cavity that is operated in a fully reflective mode and is provided with a birefringent material in a cavity between two reflectors. A first mirror, for example of about 90% reflectance and a second mirror, for example of 99% reflectance, define the cavity. A polarization transformation is applied exclusively to a resonant wavelength that is defined by the spacing of the two reflectors and by the orientation of the birefringent medium, which is preferably tunably controllable. Preferably, the entire input beam is reflected back in the direction of incidence. However the resonant wavelength component therein has experienced a polarization transformation whereby the resonant wavelength differs from the non-resonant wavelengths. This difference permits discrimination for the resonant wavelength, for example, for selective diversion using a polarization beam splitter. A number of applications are disclosed, including using a birefringent liquid crystal and tuning the apparent optical path length by electrically adjusting the birefringence. The device also is cascadable for selectively operating on certain wavelengths and diversely polarizing some wavelengths and not others. In one embodiment wherein the purpose is to separate the resonant wavelength, the input beam can be applied at 45 degrees to the slow axis of buff-oriented birefringent nematic liquid crystal. Additional variations also are possible.